The invention relates to photon detectors, particularly to ultra-violet, visible, and infra-red photon detection, and more particularly to a buffer assembly for a gamma-insensitive sensor which involves the conversion of incident optical photons into photoelectrons and subsequent amplification of these photoelectrons via generation of electron avalanches.
Photon detectors operate by converting photons into electronic signals that can be processed into pulses or images. These include devices such as photodiodes, photomultiplier tubes, vidicons, charged-coupled devices (CCD's) etc. All photon detectors are characterized by their sensitivity to photons as a function of photon energy, their ability to amplify incident photons into large electrical signals proportional to the incident photon intensity (gain), their ability to distinguish fine detail in an image (position resolution), their temporal response to incident photons (time resolution), and their inherent noise level (dark current).
Various types of photon sensing or detection devices and imaging systems using the detected photons are known in the art as exemplified by U.S. Pat. Nos. 5,032,729 issued Jul. 16, 1991 to G. Charpak; 4,853,395 issued Aug. 1, 1989 to R. R. Alfano et al.; 4,687,921 issued Aug. 18, 1987 to H. Kojola; 4,564,753 issued Jan. 14, 1986 to G. VanAller et al.; and 4,070,578 issued Jan. 24, 1978 to J. G. Timothy et al.
Optical sensors operating in ultraviolet, visible, and infra-red wavelength bands have a variety of applications. The current generation of sensors, such as exemplified above, uses various types of semiconductor focal plane arrays to detect the optical photons emitted, for example, by the combustion of fuel for propulsion, such as a various rockets and/or space vehicles. In certain applications, the optical sensors need to be capable of operating in environments, such as nuclear. One of these environments is the gamma flux emitted by fission or generated by neutron capture in the sensor and near-by materials. Ionizing events caused by these gammas, mainly via Compton, photo and pair-produced electrons, in the thin sensitive layers of the focal plane pixels, will blind the sensor once the gamma flux is sufficiently large so as to produce one or more ionizing events in each and every pixel in the time interval during which the pixels integrate the charges produced by optical photons. For certain applications this blinding gamma flux is on the order of 10.sup.8 gammas/cm.sup.2 /sec and higher.
When a single Compton electron traverses the sensitive layer of a semiconductor focal plane array pixel, which is typically 10 microns thick, it will deposit enough energy to produce on the average 10.sup.4 hole-electron pairs. An optical photon, when absorbed in this same layer, will produce only a single hole-electron pair. It is this 10.sup.4 :1 advantage of a Compton electron relative to an optical photon that enables as little as one gamma event to overwhelm the charge deposited on a pixel from all the optical photons collected in a typical sample time.
While the gamma-insensitive optical sensor of above-referenced application Ser. No. 08/011639 satisfied the prior need for an optical sensor capable of operating in a gamma flux environment, using an avalanche gas in contact with the photocathode, whereby the gammas can be rejected or distinguished from the optical photons, the avalanche gases must be chemically compatible with the photocathode materials. Thus, the sensor of the parent application was limited to choice of avalanche gases due to the chemical compatibility limitation. The present invention provides a solution to this chemical compatibility limitation by providing a buffer between the photocathode and the gas.